The soft normally open point (SNOP) is gradually applied in distribution networks due to its flexible power regulation ability. However, owing to the access of a large number of distributed generations (DG), the regulation ability of SNOP is limited by line capacity constraints. A definition of the sensitivity of SNOP to the line active power margin regulation for distribution systems is proposed in this paper as an evaluation indicator for SNOP's regulation ability, and the allocation optimization model for SNOP is established accordingly. On this basis, two evaluation indicators, namely node voltage margin and line power margin, are introduced to construct a distribution network operation optimization model with the objective of maximizing comprehensive operation margin. The model is converted to a second-order cone programming model, which is efficiently solved using MATLAB. Finally, the proposed model and solution method are validated on modified IEEE 33-node system, further indicating that the proposed method can maximize the regulating ability of SNOP, and the optimized control strategy can achieve operational safety and economy of the distribution network.